Drip pan lng tank

ABSTRACT

A container tank useful in the transportation of liquified gases at cryogenic temperatures in a tanker vessel comprises a primary liquid-tight barrier and a secondary liquid-tight barrier spaced apart from and surrounding the primary barrier and including a lower drip pan portion which is structurally integrated with the primary barrier and an upper splash shield portion which is structurally independent of but may be mechanically fastened to the primary barrier, i.e., the splash shield may be supported by the primary barrier but not structurally integral with said primary barrier.

United States Patent Cowles et al.

1451 Sept. 19, 1972 DRIP PAN LNG TANK [72] Inventors: Walter C.Cowles,Stamford, Conn.;

Frank J. larosi, Middletown, NJ.

[73] Assignee: Esso Research and Engineering Company 221 Filed: Feb. 27,1970 [21] Appl. No.: 15,054

52 us. CL; ..220/9 LG, 114 74 A, 62/45 51 Int. Cl. ..B65d 25/18 [58]Field of seal-ell ..62/45, 55; 114/74 A; 220/9111 [56] I ReferencesUNITED STATES PATENTS 3,312,076 4/1967 I Clarke etal. ..62/55 3,347,402

10/1967 Foremanetal ..114/74AX Primary Examiner-Meyer Perlin AssistantExaminer-Ronald C. Capossela Attorney-Manahan and Wright and Jay Simon[57] ABSTRACT A container tank useful in the transportation of liquifiedgases at cryogenic temperatures in a tanker vessel comprises a primaryliquid-tight barrier and a secondary liquid-tight barrier spaced apartfrom and surrounding the primary barrierand including a lower drip panportion which is structurally integrated with the primary barrier and anupper splash shield portion which is structurally independent of but maybe mechanically fastened to the primary barrier, i.e., the

' splash shield may be supported by the primary barrier but notstructurally integral with said primary barrier.

10 Claims, 5" Drawing Figures PKTENTEDSEH 1m 3,692,205

sum 1 or 2 v Fig. 2

Walter C. Cow/es Frank J. Iarossi INVENTORS jet BY ATTORNEY SHEET 2 BF 2PNENTEDSEP 19 I972 Walter C. Cow/es Frank J. Iarossi VENTORs J1 A BY 7v\ ATTORNEY DRIP PAN LNG TANK FIELD OF THE INVENTION primary barrier andan upper portion, referred to as a splash shield, which is structurallyindependent of but may be mechanically fastened to the primary barrier.

BACKGROUND OF THE INVENTION In the transportation of liquefied gases atcryogenic temperatures, e.g., methane liquefies at -259F., and atatmospheric pressure in tanker vessels whose hulls are constructed ofordinary carbon steel, it is extremely important to prevent the lowtemperature liquid from coining into contact with the hull. Thus,ordinary carbon steel is quite susceptible to embrittlement at lowtemperatures leading to the fracture of the hull plate structure. Oneapproach to this problem has been to construct a fully redundantdouble-wall tank, i.e., wherein each wall is liquid-tight and ofsufficient structural integrity to withstand the entire dynamic andstatic loads of the liquid contents of the tank. This technique wasbelieved to provide the maximum degree of safety but only at high costs(since the second or outside barrier was only useful in the event ofrupture of the inner or primary barrier). Of course, suitable insulatingmeans were applied to the exterior of the secondary barrier to maintainthe cryogenic temperature.

On the other hand, costs could be minimized by utilizing only a singlewall container on the theory that the only type of failure that wouldoccur would be a catastrophic failure, e.g., as would occur on acollision, and the hull would be ruptured anyway. However, this proposaldoes not take into account leaks or seepage from the tank due tofatiguecracks and also does not meet the safety regulations of many countries,thereby increasing the liability risks of the ship operators.

Other prior art tank designs have employed a doublewall constructionwherein the primary and secondary barriers are completely integralstructurally. This type of construction is very costly and has beenfound to be overdesigned in many respects and relatively difficult tomanufacture.

Since technology advances have shown that a failure is most likely to beminor and not of a catastrophic nature, it is generally only necessaryto provide a greater degree of protection against leaks with a lesseramount of protection for major failures. By the practice of thisinvention, however, the advantages of a secondary barrier containing aspray shield and drip pan are maintained and the disadvantages areovercome.

SUMMARY OF THE INVENTION In accordance with this invention, therefore, aliquefied gas container, particularly suitable for use in a tankervessel, is provided which has more than adequate reliability for meetingthe highest safety standards but retains the economical features ofsingle-wall tank construction. Thus, a container is provided whichcomprises a primary barrier, or inner wall, of sufficient strength andstructural design to retain the enclosed cryogenic liquid, i.e., towithstand the static and dynamic loads produced by the liquid, andspaced apart from and surrounding the primary barrier is a liquid-tightouter wall, or secondary barrier, of a thickness generally thinner thanthe primary barrier and including a lower drip pan portion which isstructurally integral with the primary'barrier and an upper spray shieldportion which is structurally independent of but may be mechanicallyfastened to the primary barrier in such a manner as to form a secondaryspraytight barrier. The purpose of the secondary wall is not to retainthe entire static and dynamic loads of the liquid as in the integraldouble-wall tank of the prior art. Rather, the purpose of the secondarywall is to act as a'liquid-tight receptacle surrounding the primarywallso that any small degree of seepage through the primary wall, e.g.,due to fatigue cracking, etc., would not come in contact with the shipshull and, furthermore, could be collected and pumped out of the spacebetween the walls.

Now, the drip pan portion is structurally integrated with the primarybarrier and extends the full bottom of the primary barrier and partiallyup the sides of' that barrier. Thus, while this portion will be subjectto the same stresses as the primary barrier, in this invention thedistance between the barriers will be such that failures in the primarybarrier cannot propagate to the drip pan. This distance can be readilyestimated or calculated by one skilled in the art knowing the materialsof construction, operating conditions, and design features, all of whichare given herein. Consequently, by simply increasing the depth betweenthe primary and secondary barriers to overcome crack propagation, aserious disadvantage of prior art proposals is eliminated. Additionally,however, several other distinct advantages are gained. Since the spacebetween the barriers will now be on the order of 3 feet, this area isnow accessible-for inspection, maintenance, and construction, therebyeliminating one-side welding problems and readily allowing visibleinspection of the primary barrier.

The upper portion of the secondary barrier, i.e., the spray shield, isformed by a composite of materials arranged to provide a spray-tightbarrier and fastened to the primary wall such that each panel is fullyrestrained at only one point. Consequently, stresses on this portionwill be minimized and the spray shield will not be affected by stresseson the primary wall. Also, removal of the upper portion of the secondarybarrier will be relatively easy maintenance maintenace work is requiredin the space between the primary and secondary barriers.

As in prior art techniques, the insulation may be applied to the outsideof the secondary barrier so that both the primary and secondary barrierswill operate at substantially the same temperatures, i.e., there will beno substantial thermal gradient between the primary and secondarybarriers. This location for the insulation eliminates the need forproviding any elaborate differential expansion compensation meansbetween the barriers which might otherwise be required. In addition, thetransporting vessel need not contain an inner hull to support the sprayshield or insulating.

DRAWING DESCRIPTION This invention will be more fully understood byreference to the following description and drawings wherein:

FIG. 1 is a cross-sectional view of the container as it would be mountedin a typical double-hull tanker vessel;

FIG. 2 is a fragmentary cross-sectional view taken along line 2-2 ofFIG. 1 looking in the direction of the arrows;

FIG. 3 is an enlarged fragmentary cross-sectional view showing means forremoving liquid from the space blemlreen the two walls and thesupporting of the spray s ie d;

FIG, 4 is a perspective representation of the spray shield with itsinsulation and the supporting structure on the primary barrier;

. FIG. 5 is a top plan view of the container showing a keyingarrangement.

Referring to the drawings in particular, FIGS. 1 and 2 show the shipshull in schematic cross section to include an outer steel hull portion12 and an inner steel hull portion 14 maintained in spaced relationshipwith conventional structural plates 16. Within the hold of the hull thisdefined is an insulated cargo container generally indicated at 18supported at its bottom by a load-bearing insulation material such aswooden bearing strips 20.

The insulated cargo container 18 includes an inner wall or primarybarrier 22 made structurally rigid by a plurality of internal web platesor bulkheads designated 24, which effectively divide the container intoa pluralityof compartments. These bulkheads run longitudinally as wellastransversely within the confines of the inner wall 22 and may include aplurality of cutout portions bounded by reinforcing flanges, such asdisclosed in the US. Pat. No. 3,312,076.

Extending longitudinally and, if desired, transversely as well, inegg-crate fashion, along the outside of the primary tank 22 are aplurality of T-shaped stringers or spacers 30. The T may be welded tothe exterior of the primary tank 22 and to the interior of the secondarytank (drip pan portion 32) by a plurality of structural weld beads 38(shown in FIG. 3). In previous designs, the space between tanks 22 and32 was insufficient to permit welding to the interior of drip pan 32.This necessitated welding the drip pan to the spaces on the exterior oroutside surface of the drip pan. Such a procedure is structurally muchless preferred than the welding permitted in this invention. In one formof the invention, the primary barrier may comprise a low temperatureconstruction metal such as aluminum and its alloys, stainless steel, 9percent nickel steel, and the like, but preferably aluminum, and may beabout onehalf to three-fourths inch in thickness depending upon thedesign load and stress. The drip pan 32 is generally of a lesserpredetermined thickness than the primary tank, again depending upon theworking thickness for specified static or dynamic loading. A layer ofinsulation 40 is applied to the exterior surface of the outer wallportions of 32. The insulation 40 is preferably of a polyurethane foam,but may also be a polystyrene foam or any other suitable insulatingmedium, which may be applied by spraying or, in the alternative, fixedto the container in a series of interlocking panels. An air space 42,which may be in the order of 2 feet or so, is provided between theoutside of the insulation 40 and the interior wall 14 of the tankerhull. The air space 42 is of sufficient size to permit periodic visualinspection of the exterior of the insulation.

Referring to FIG. 3 in detail, it may be seen how each of the stringers30 includes a plurality of apertures 44 which are preferably in verticalalignment and permit the downward passage therethrough of a conduit 46having a lower end which will be understood to terminate in the lowerportion of the space between the inner and outer walls of the tank.Conduit 46 is-connected to a pump 48 of conventional construction sothat any seepage of liquid from the interior of the container into thespace between the inner and outer walls may be readily removed by theaction of the pump 48.

Referring to FIG. 3 and FIG. 4, a substantially Z- shaped type connector74 may be welded to spacer 30 and/or drip pan 32 for supporting thespray shield. Now it is important that there be no structural weldsbetween the spray shield and the primary barrier or between the sprayshield and the drip pan. Therefore, mechanical connections, e.g.,rivets, studs or nonstructural welds are employed to position the sprayshield. T-stiffeners 60 running vertically along the exterior of primarybarrier 22 can be used to support, by mechanical connections, and spacethe spray shield from the primary barrier as well as giving structuralintegrity to the primary barrier. These stiffeners may also be locatedinteriorly of the primary wall (and firring strips may be used on theexterior for mounting the spray shield) but are preferably located, asshown, for ease of construction. When the stiffeners are interior of theprimary barrier, the previously-described interior sections of the tankmay not be necessary although they are often preferred for reducing thetendency of the liquid to slosh about due to ship movement.

Mounted on the Z-shaped connector 74 (which forms a part of thesecondary liquid-tight barrier) is the spray shield comprising analuminum sheet or other impervious material, e.g., plywood 62, whichacts to deflect leaks in the upper portion of the primary barrierdownwardly into the drip pan. The laterally extending flanges -70 assistin accomplishing the desired deflection. Since connector 74 is incontact with internal sheet 62 it forms a thermal short and thus, it isnecessary to overlay 74 with the insulation 40. This shield can be quitethin, e.g., on the order of one-sixteenth to one-eighth inch. The sprayshield has a sandwich type insulation system with layers of foaminsulation 64 mounted on the spray shield 62and sandwiching a plywoodstructural barrier 66. The exterior of the spray shield insulationcomprises preferably a single sheet of aluminum 68 which may be caulkedor otherwise made tight at all joints with silicone or rubber beads. Analternate arrangement may comprise a shingle type construction with adownward overlap. The liquid-tight spray shield assembly thus provides aliquefied gas shield on its interior surface and a water shield on itsexternal surface. The latter shield is important in that it providesprotection for the insulation from water leaks in the bull or fromballast tanks (not shown) located between the inner and outer hulls atspace 43 (shown in FIG. 1 The sandwich foam insulation is utilized sothat there will not be a thermal bridge between the inner spray shield62 and the outer aluminum sheathing 68. Thus, foam panels 64 can bemounted with metallic fasteners on plywood spacer 66 at staggered pointswithout creating a thermal bridge. The insulation around the entire tankstructure is thereby maintained without compromising its effectiveness.

Turning to FIG. 5, one exemplary manner in which the entire insulatedcargo tank could be secured and located to the hull structure may beseen. As previously pointed'out, the static load of the insulated cargocontainer is transmitted to the ships hull through the loadbearinginsulation 20, e.g wood beams, at the bottom of the tank. The mountingof the novel insulated cargo container is such that it permits thermalexpansion and contraction thereof. In the typical schematic arrangementshown, the top as well as the bottom walls (not shown) of the exteriorwall of the cargo container 18 are provided with a keying arrangementcomprising a key 76 extending longitudinally and two transverselydisposed keys 78, which may be structurally integral with the container18. Each key cooperates with a pair of guide blocks schematically shownin FIG. 1. at 79 to form A KEYWAY. The exterior sides of the keys aresituated relative to the interior sides of the guide blocks, so thateach key is located in the keyway in a slip-fit relationship. The guideblocks are conventionally secured in turn to the inner hull wall 14. Inthis manner of mounting of the cargo tank, substantially the entire loadof the cargo within the container is transmitted through the bearingstrips to the hull structure. The keying arrangement restrains the tankfrom undesirable shifting within the tanker hold. Although one type ofkeying system has been disclosed it will be apparent to those skilled inthe art that other suitable types of keying arrangements may beemployed.

Referring again to FIG. 1, the upper portion of the hull structure 12 ofthe tanker forms the deck of the tanker, or the support for the deck.Additionally, the outside surface of the inner hull 14 at the upperportion of the tank is provided with a plurality of longitudinalT-shaped stiffener members 52 attached to the inner hull and extendinginto ballast space 43. Also shown schematically in FIG. 1 are top andbottom keys 76 which are constrained in guide blocks (not shown).

In another embodiment of this invention, the spray shield together withthe drip pan need not completely envelop the primary barrier. Thus, forexample, the spray shield can be omitted, if desired, from the topportion of the primary barrier. Any leak at the top of the primarybarrier would cause liquid to trickle down along the exterior of theprimary barrier and into the drip pan. However, it is generallypreferred to have the spray shield continuous from one side of the drippan to the other side and thereby form a continuous secondaryliquid-tight barrier fully enclosing the primary liquidtight barrier.

While a specific embodiment of this invention has been described andshown otherwise detail to illustrate the application of the inventiveprinciples, it will be understood that the invention may be embodiedtogether without departing from such principles.

What is claimed is:

1. A container for liquefied gases at cryogenic temperatures for use ina tanker which comprises, in combination: a primary tank of sufficientstructural strength to contain the liquefied gas, said primary tankcontaining internal structural means fixed to the interior wallsthereof; a larger liquid-tight secondary enclosure substantially similarin shape to said primary tank and completely surrounding said primarytank, said secondary enclosure containing a lower portion ofpredetermined thickness less than the thickness of the primary tank butsufficient to retain small quantities of the liquefied gases that mayleak through said primary tank; thermally conductive structural meansfor maintaining said lower portion in spaced relationship andstructurally integral with said primary tank, said structural meanshaving sufiicient depth to prevent failures in the primary tank frompropagating to said lower portion; said secondary enclosure alsocontaining a liquidtight upper portion for deflecting seepage throughupper portions of the primary tank into said lower portion; means formechanically fastening said upper portion of said secondary enclosure tosaid lower portion and said primary tank solely for support and topermit relatively quick assembly and disassembly while maintaining saidupper portion structurally independent of said primary tank and saidlower portion; said lower portion being spaced at a first distance fromthe sides of said primary tank, and said upper portion being spaced at asecond distance which is less than said first distance from said primarytank.

2. A container in accordance with claim 1 wherein thermal insulationmeans is secured externally to the entire surface of the upper and lowerportions of said secondary enclosure.

3. A container in accordance with claim 1 wherein keying means areattached to said container for operable cooperation with the structureof said tanker for permitting relative movement therebetween, saidkeying means including at least a top and bottom longitudinallyextending key-keyway.

4. A container in accordance with claim 3 wherein a plurality oftransversely extending wooden bearing members are located between thebottom portion of said lower portion of said secondary enclosure and thehull of said tanker on either side of the bottom keykeyway, whereby theentire normal static load of said container and liquefied gas cargo istransmitted to the bottom hull of said tanker.

5. A container in accordance with claim 2 wherein the thermal insulationmeans applied to said upper portion of said secondary enclosurecomprises a double layer of foam insulation sandwiching a thermallynonconductive member, said member acting to support the foam insulation,and an outer metallic covering over the outer layer of foam insulationwhich covering shields the foam insulation from leaks in the hull ofsaid tanker.

6. The container in accordance with claim 1 wherein the secondaryenclosure only substantially surrounds the primary tank.

7. A container in accordance with claim 1 wherein the mechanicalfastening means includes a connector operably secured with said lowerportion and said upper portion of said secondary enclosure andconstructed and arranged for deflecting leaks in the upper containinginternal structural means fixed to the interior walls thereof; a largerliquid-tight secondary enclosure substantially similar in shape to saidprimary tank and completely surrounding said primary tank, saidsecondary enclosure containing a lower portion of predeterminedthickness less than the thickness of the primary tank but sufficient toretain small quantities of the liquefied gases that may leak throughsaid primary I tank; thermally conductive structural means formaintaining said lower portion in spaced relationship and structurallyintegral with said primary tank, said structural means having sufficientdepth to prevent failures in the primary tank from propagating to saidlower portion; said secondary enclosure also containing a liquidtightupper portion for deflecting seepage through upper portions of theprimary tank into said lower portion; means for mechanically fasteningsaid upper portion of said secondary enclosure to said lower portion andsaid primary tank solely for support and to permit relatively quickassembly and disassembly while keeping said upper portion structurallyindependent of said primary tank and said lower portion; top and bottomkeying means operably disposed between said container and the adjacentstructure of said tanker for restraining said container from undesirableshifting while permitting relative movement therebetween, each of saidtop and bottom keying means including a key-keyway extendinglongitudinally along the centerline of the container and a pair oftransverse keykeyways located on either side of the longitudinalkeykeyway substantially at the midpoint thereof.

10. A container in accordance with claim 9 wherein a plurality oftransversely extending load bearing insulation members are spacedlongitudinally along the bottom of said container on either side of thelongitudinally extending bottom key-keyway.

1. A container for liquefied gases at cryogenic temperatures for use ina tanker which comprises, in combination: a primary tank of sufficientstructural strength to contain the liquefied gas, said primary tankcontaining internal structural means fixed to the interior wallsthereof; a larger liquid-tight secondary enclosure substantially similarin shape to said primary tank and completely surrounding said primarytank, said secondary enclosure containing a lower portion ofpredetermined thickness less than the thickness of the primary tank butsufficient to retain small quantities of the liquefied gases that mayleak through said primary tank; thermally conductive structural meansfor maintaining said lower portion in spaced relationship andstructurally integral with said primary tank, said structural meanshaving sufficient depth to prevent failures in the primary tank frompropagating to said lower portion; said secondary enclosure alsocontaining a liquid-tight upper portion for deflecting seepage throughupper portions of the primary tank into said lower portion; means formechanically fastening said upper portion of said secondary enclosure tosaid lower portion and said primary tank solely for support and topermit relatively quick assembly and disassembly while maintaining saidupper portion structurally independent of said primary tank and saidlower portion; said lower portion being spaced at a first distance fromthe sides of said primary tank, and said upper portion being spaced at asecond distance which is less than said first distance from said primarytank.
 2. A container in accordance with claim 1 wherein thermalinsulation means is secured externally to the entire surface of theupper and lower portions of said secondary enclosure.
 3. A container inaccordance with claim 1 wherein keying means are attached to saidcontainer for operable cooperation with the structure of said tanker forpermitting relative movement therebetween, said keying means includingat least a top and bottom longitudinally extending key-keyway.
 4. Acontainer in accordAnce with claim 3 wherein a plurality of transverselyextending wooden bearing members are located between the bottom portionof said lower portion of said secondary enclosure and the hull of saidtanker on either side of the bottom key-keyway, whereby the entirenormal static load of said container and liquefied gas cargo istransmitted to the bottom hull of said tanker.
 5. A container inaccordance with claim 2 wherein the thermal insulation means applied tosaid upper portion of said secondary enclosure comprises a double layerof foam insulation sandwiching a thermally nonconductive member, saidmember acting to support the foam insulation, and an outer metalliccovering over the outer layer of foam insulation which covering shieldsthe foam insulation from leaks in the hull of said tanker.
 6. Thecontainer in accordance with claim 1 wherein the secondary enclosureonly substantially surrounds the primary tank.
 7. A container inaccordance with claim 1 wherein the mechanical fastening means includesa connector operably secured with said lower portion and said upperportion of said secondary enclosure and constructed and arranged fordeflecting leaks in the upper portion of said primary tank downward intosaid lower portion.
 8. A container in accordance with claim 5 whereinsaid outer metallic covering comprises a shingle-type arrangement with adownward overlap.
 9. A container for liquefied gases at cryogenictemperatures for use in a tanker which comprises, in combination: aprimary tank of sufficient structural strength to contain the liquefiedgas, said primary tank containing internal structural means fixed to theinterior walls thereof; a larger liquid-tight secondary enclosuresubstantially similar in shape to said primary tank and completelysurrounding said primary tank, said secondary enclosure containing alower portion of predetermined thickness less than the thickness of theprimary tank but sufficient to retain small quantities of the liquefiedgases that may leak through said primary tank; thermally conductivestructural means for maintaining said lower portion in spacedrelationship and structurally integral with said primary tank, saidstructural means having sufficient depth to prevent failures in theprimary tank from propagating to said lower portion; said secondaryenclosure also containing a liquid-tight upper portion for deflectingseepage through upper portions of the primary tank into said lowerportion; means for mechanically fastening said upper portion of saidsecondary enclosure to said lower portion and said primary tank solelyfor support and to permit relatively quick assembly and disassemblywhile keeping said upper portion structurally independent of saidprimary tank and said lower portion; top and bottom keying meansoperably disposed between said container and the adjacent structure ofsaid tanker for restraining said container from undesirable shiftingwhile permitting relative movement therebetween, each of said top andbottom keying means including a key-keyway extending longitudinallyalong the centerline of the container and a pair of transversekey-keyways located on either side of the longitudinal key-keywaysubstantially at the midpoint thereof.
 10. A container in accordancewith claim 9 wherein a plurality of transversely extending load bearinginsulation members are spaced longitudinally along the bottom of saidcontainer on either side of the longitudinally extending bottomkey-keyway.